Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector

From: Dmitry Safonov <dima@arista.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Vitaly Kuznetsov" <vkuznets@redhat.com>,
	linux-kernel@vger.kernel.org,
	"Prasanna Panchamukhi" <panchamukhi@arista.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Borislav Petkov" <bp@alien8.de>,
	"Cathy Avery" <cavery@redhat.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Ingo Molnar" <mingo@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
	"Mohammed Gamal" <mmorsy@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Roman Kagan" <rkagan@virtuozzo.com>,
	"Sasha Levin" <sashal@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	devel@linuxdriverproject.org, kvm@vger.kernel.org,
	linux-hyperv@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] x86/hyperv: Disable preemption while setting reenlightenment vector
Date: Fri, 14 Jun 2019 15:28:37 +0100	[thread overview]
Message-ID: <d0ea735e-487e-8205-9415-8708a686ede9@arista.com> (raw)
In-Reply-To: <20190614122726.GL3436@hirez.programming.kicks-ass.net>

On 6/14/19 1:27 PM, Peter Zijlstra wrote:
> On Fri, Jun 14, 2019 at 12:50:51PM +0100, Dmitry Safonov wrote:
>> On 6/14/19 11:08 AM, Vitaly Kuznetsov wrote:
>>> Peter Zijlstra <peterz@infradead.org> writes:
>>>
>>>> @@ -182,7 +182,7 @@ void set_hv_tscchange_cb(void (*cb)(void))
>>>>  	struct hv_reenlightenment_control re_ctrl = {
>>>>  		.vector = HYPERV_REENLIGHTENMENT_VECTOR,
>>>>  		.enabled = 1,
>>>> -		.target_vp = hv_vp_index[smp_processor_id()]
>>>> +		.target_vp = hv_vp_index[raw_smp_processor_id()]
>>>>  	};
>>>>  	struct hv_tsc_emulation_control emu_ctrl = {.enabled = 1};
>>>>  
>>>
>>> Yes, this should do, thanks! I'd also suggest to leave a comment like
>>> 	/* 
>>>          * This function can get preemted and migrate to a different CPU
>>> 	 * but this doesn't matter. We just need to assign
>>> 	 * reenlightenment notification to some online CPU. In case this
>>>          * CPU goes offline, hv_cpu_die() will re-assign it to some
>>>  	 * other online CPU.
>>> 	 */
>>
>> What if the cpu goes down just before wrmsrl()?
>> I mean, hv_cpu_die() will reassign another cpu, but this thread will be
>> resumed on some other cpu and will write cpu number which is at that
>> moment already down?
>>
>> (probably I miss something)
>>
>> And I presume it's guaranteed that during hv_cpu_die() no other cpu may
>> go down:
>> :	new_cpu = cpumask_any_but(cpu_online_mask, cpu);
>> :	re_ctrl.target_vp = hv_vp_index[new_cpu];
>> :	wrmsrl(HV_X64_MSR_REENLIGHTENMENT_CONTROL, *((u64 *)&re_ctrl));
> 
> Then cpus_read_lock() is the right interface, not preempt_disable().
> 
> I know you probably can't change the HV interface, but I'm thinking its
> rather daft you have to specify a CPU at all for this. The HV can just
> pick one and send the notification there, who cares.

Heh, I thought cpus_read_lock() is more "internal" api and
preempt_diable() is prefered ;-)

Will send v2 with the suggested comment and cpus_read_lock().

-- 
          Dima